• Bipolar patients taking lithium carbonate were classified as rapid-cycling or non—rapid-cycling based on whether they had ever experienced four or more affective episodes in a 12-month period. Overt hypothyroidism was found in 12 (50.7%) of the 24 rapid-cycling patients and in none of the 19 non—rapidcycling patients. Elevated thyroid-stimulating hormone levels were present in 92% of the rapid-cycling group v 32% of the non—rapid-cycling group. Abnormalities of the hypothalamic-pituitary-thyroid axis, some of which may become apparent only during treatment with lithium carbonate, appear to interact with a predisposition to bipolar illness to produce rapidcycling. These overt and covert abnormalities may help explain the reported efficacy of thyroid in treating "periodic catatonia" and rapid-cycling.